A device for measuring the steam flux from a surface is known from U.S. Pat. No. 7,631,538 B2. More specifically, the surface is human skin whose water loss rate is to be determined. A first end of a small tube is put on the surface. By means of atmospheric humidity sensors that are spaced from one another in the extending direction of the small tube, the moisture gradient in the small tube is measured. Then, the steam flux away from the surface can be calculated with the obtained data.
A so-called porometer for measuring the transpiration of plant leaves is known from U.S. Pat. No. 4,312,218 A. A leaf to be measured is enclosed in a chamber. Then, a current of dry air into the chamber is controlled in a way that the atmospheric humidity in the chamber remains constant. The transpiration of the leaf can eventually be determined from the flow speed of the air.